OldIHC

I found a data book for leaf springs which might be helpful in finding springs for obsolete parts.
http://www.springresearch.org/pdf/volume_25.pdf
I would suggest downloading a copy as I am not sure the link will always be active.
edit: If you replace the volume number in the above link, you can access info back to 1932!

I/m not sure about the big trucks, the guys at St Louis Spring got it together on the lite line over the phone. Good prices too. And fit.

When I was doing heavy-haul, I wore out the front springs on my 9670. I replaced the 12000lb front springs with 16,000lb set. It allowed me to drop the air on the pusher, putting 13-14K on the steer for making off ramps and tight turn, without bottoming out. Bobtail, I was running 12,400 on the steer, so would be maxed out before I even put a trailer on. To this day the front springs are going strong, and rides well.
At that time, I had access to a book that covered IHC. This one gives measurements and spring rates which can be helpful in selection of a non standard replacement.

You know the big stuff, I don't! use to drive the big trucks with booms on them, but the company sent them out when something went wrong. When the guy asked what P/N was on my springs and how many, I was surprised, so I told him. Had them in 3 days.